Earlier today on our sister site Inside Mobile Apps, we reported that Apple may have changed the ranking algorithm for the iOS app store, favoring active usage in addition to downloads. A number of the bigger pay-per-install networks, which help developers break into the top of the charts through buying downloads, noticed strange shifts in the rankings last week that seemed to favor older apps with large active user bases like Netflix and Pandora.

While Apple hasn’t publicly confirmed the changes, the big beneficiary has been Facebook, which scaled the charts to number #1 after having mostly lingered between #10 and #20 for the last 16 months.

Facebook’s iPhone app has been adding monthly active users at a clip of just over 600,000 users a week. It almost certainly has the largest daily active user base of any third-party app on the iOS platform with 39.5 million users opening it every day, according to AppData. Just for a frame of reference: if Apple is getting close to having cumulatively sold 200 million iOS devices as analysts estimate, the app gets opened on at least one out of every five iOS devices every day.

Facebook for iPhone Statistics

While adding 600,000 users a week sounds like a lot, it’s actually small compared to what we’ve heard top-ranked free apps would pull in before the algorithm changes. The very top three places often represent more than 300,000 downloads a day while almost all of the top 10 slots usually do at least 100,000 a day, according to conversations we’ve had with developers who have held those ranks.

All of this goes to suggest that Apple is favoring active usage more relative to downloads. There isn’t anything that Facebook appears to have done to independently boost its app; the company did release an update on April 4 — but that was about a week and half before its rise. Facebook is a remarkably sticky app with roughly 55 percent of its monthly active users opening up the app every day.

Here are some additional growth charts, showing a steady, linear trajectory for the app. Like we said above, we can’t identify any growth spikes on Facebook’s side, so it does appear to be a change on Apple’s side. Overall, the extra visibility will likely help convince users who haven’t already downloaded the app to install it. We’ll be watching AppData to see if the changes produce any measurable bump in the number of monthly actives the app is adding.